One prior computer system had an interrupt controller. The interrupt controller was coupled to peripheral components and also coupled to a processor. When a peripheral component needed the processor to perform an operation, it transmitted an interrupt request signal to the interrupt controller. The interrupt controller in turn then transmitted an interrupt request to the processor. The processor then determined the source of the interrupt request and serviced the interrupt. The prior computer system however, lacked power management features. When the prior computer system was left on, it remained in the fully operational mode despite long periods of inactivity.
Recently, in an effort to reduce power consumption by computer systems, the United States Environmental Protection Agency (EPA) established guidelines encouraging design and manufacture of computer systems which when left idle for an extended period, only consumed a specified amount of power or less. In conforming to the guidelines, the computer system must be placed in a low power mode after a specified idle period. One previously unrecognized problem is that when restoring the computer system after some activity has occurred, the interrupt request generated as a result of the activity may arrive before the processor has switched back to the fully operational mode. The computer system will then attempt to service the interrupt in the low power mode resulting in an inability to service the interrupt or in an inefficient servicing of the interrupt.
Thus, what is needed is a circuit that ensures that the computer system switches to a fully operational mode before servicing an interrupt.